The invention relates to a method for generating more uniform rotation with respect to mean time of an effective space vector of the stator current of asynchronous rotating machinery. The machinery is operated by a d.c. current intermediate circuit-fed inverter with phase sequence quenching. The ignition of each inverter is controlled by a thyristor. The space vector rotates cyclically in the direction of rotation of the machinery-dependent commutation sequence of six equidistant natural space vectors. At low machine speed, a pulse-like commutation between two adjacent natural space vectors occurs.
The inverter part of a current intermediary circuit frequency converter that has phase sequence quenching that feeds the stator phases of asynchronous rotating machinery is shown in FIG. 1. The frequency converter is fed from a d.c. current intermediary circuit with an intermediary circuit current i.sub.d from choke L.sub.d. Each phase of the asynchronous machinery is connected to the center of an inverter branch the upper and lower half of which each consist of one thyristor and a series diode. Thus, the phase R, for example, is connected to the center of the two series circuits formed by thyristor T1 and diode D1 and diode D4 and thyristor T4, respectively. All thyristors in the upper and lower inverter halfs, e.g. the thyristors T1, T3, T5 and T4, T6, T2, respectively, are cumulatively connected via commutation capacitors C1, C3, C5, and C2, C4, C6, respectively. The current conducting time of each thyristor in this self-commutated converter can be set in a given manner by igniting another thyristor of the same inverter half at the appropriate time. The thyristor conducting current is simultaneously and abruptly cut off due to the charge of the commutation capacitor between the two thyristors.
To maintain the intermediate circuit current I.sub.d free of interruptions, one thyristor in either the upper inverter half or one in the lower inverter half is always switched on. Further, the thyristors corresponding to a commutation series cyclically alternate and take turns in the upper inverter half and lower inverter half in conducting the intermediate circuit current depending on the direction of rotation of the machine. The output of the inverter thus approximates a three-phase system that generates a settled frequency. The currents generated in the stator phases of connected asynchronous rotating machinery are represented by a "complex space vector" as is customary for the description of processes in electrical machinery. This complex space vector produces a vector diagram corresponding to FIG. 2. The space vector Z of the stator current rotates around the complex plane depending on the direction of commutation. The vector can assume one of six equidistant fixed positions, called natural space vectors, the points of which form an regular hexagon. These are designated in FIG. 2 as I to VI. The indications 1, 2 to 6, 1 correspond to the associated inverter thyristors. If, for example, the thyristor pair T4, T5 conduct current, the natural space vector assumes position IV. If the thyristor T4 is made nonconductive through ignition of thyristor T6, the space vector Z jumps to position V.
A more "uniform rotation" of the space vector of the stator current in the complex plane produces a better harmonic oscillation of the torsional moment generated by the asynchronous rotating machinery. The ideal case occurs when the asynchronous rotating machinery is operated on a symmetrical network with sineusoidal currents. The space vector then revolves continuously on a circular path and assumes all possible positions between the natural space vectors. Feeding asynchronous machinery at medium or high speed via an inverter according to FIG. 1 produces a good approximation to the ideal case. In particular, the presence of a sufficient centrifugal moment of the machinery causes the discrete "rotation" of the current vector among the six possible positions of the natural space vector to become indiscernable. The concentric running quality of the machinery is normally sufficient.
However, a low, near zero speed of rotation requires additional measures to improve the concentric running properties. The operation is normally changed to "pulsed operation" in which the space vector is switched back and forth with a pulse duty factor that is supplied through a modulation process. The pulse operation produces a space vector that is angle-dependent and lies between two of the natural space vectors. The "effective" current space vector resulting from this "mixture" can thus theoretically assume any possible intermediate position between the natural space vectors so as to rotate "quasi continuously" in mean time. An example of this method is disclosed in the German patent document No. 2,236,763.
Nevertheless, pulsed operation causes an interfering instantaneous voltage ripple to occur, especially at low machine speed. Its cause can be found in that the effective space vector of the stator current jumps across "forbidden angle zones" around the natural space vector positions. In FIG. 2, these forbidden angle zones have a magnitude of 2 .DELTA..phi. and are indicated by a dash-dot line. For example, upon reaching boundary A of the forbidden angle zone around position II, space vector Z must jump the natural space vector to boundary B before it can again travel quasi continously through the succeeding angle region C to the next forbidden angle region around the natural space vector III. The width of the forbidden angle zones is determined by the so-called inverter minimum time which indicates the least permissable time distance between two commutations in the inverter. The commutation capacitors reverse their charge after ignition of an inverter thyristor.
The processes occurring during a commutation in a self-commutated inverter with phase sequence quenching is explained in detail, for example, in the ETZ-A, Volume 96 (1975), No. 11, pages 520-523. The commutation process is completed only after the particular commutation capacitors have reached their full charge state with reversed signs, e.g., all charge-up processes have been completed. The next ignition of a thyristor must be delayed at least by this length of time. If, for example, according to the representation in FIG. 2, the effective space vector Z, generated through pulsed commutation between the inverter thyristors T1 and T3, is intended to travel toward the natural space vector in position II, the setting ratio of the two thyristors is displaced in favor of thyristor T3. Accordingly, the duration of the setting pulse for thyristor T1 becomes increasingly shorter. These fall be1ow the inverter minimum time the space vector Z after briefly lingering in position A jumps to position II. To continue the travel of the current vector Z with thyristor T3 switched on, thyristors T2 and T4 alternate pulse-like between themselves as current conductors. The space vector Z, after lingering briefly, jumps initially into position B. Only then do the switching pulses for the thyristor T4, which do not fall below the inverter minimum time, occur. The on-ratio of the two thyristors taking part in a given "mixture" for generating the effective space vector Z in an intermediate position with respect to the natural space vectors is limited by the inverter minimum time.
lncreasing the frequency of the pulsed commutation called pulse frequency, makes the rotation of the effective space vector in the intermediate position more uniform. However, it also enlarges the forbidden angle regions. With greater pulse frequencies the switching pulses per pulse period are more finely graded, but early switching pulses occur when the effective space vector approaches the natural space vector. The switching thus falls below the minimum time and must be suppressed.
In order to decrease the width of the forbidden angle zones, European patent application No. 86/108,484.6 suggests an "angle-sensitive pulse frequency modulation". A large value is selected for the pulse frequency if the effective space vector is approximately in the middle of the permissable angle region between two natural space vectors. If, however, the effective space vector is in the vicinity on the positions of a natural space vector, the pulse frequency is lowered. The widths of the forbidden angle regions are thus reduced markedly. In this last procedure, the width of the forbidden angle zones is predetermined by the minimum time of the inverter even if, through the pulse frequency decrease in the marginal regions, the occurrence of switching pulses falling below the minimum time can be compressed to a region closer to the natural space vectors.